Astronomy’s Magnitude Scale

Posted by Kelly on May 27th, 2013

Solar Eclipse

At magnitude -26.7, the Sun is too bright to look at without optical aid. Credit: ESO

In astronomy, the brightness of objects is measured by a term called “magnitude”. The lower the number, the brighter the object. The category of first magnitude objects are those that range from magnitude 0.51 to 1.50, with second magnitude 1.51 to 2.50, and so forth. Certain objects are even brighter than first magnitude, peaking in negative numbers at their brightest.

Without optical aid, the human eye can see objects to around 6th magnitude. There are ten natural solar system objects you can see with the unaided eye. (This does not count manmade objects, such as satellites and the International Space Station.)

Here are the top 10 brightest solar system objects, in order from brightest to dimmest:

  1. The Sun. No surprise here. The Sun shines at magnitude -26.7. Technically, the Sun can’t even be looked at without optical aid when it’s at its brightest. Hazy days at sunrise and sunset are the best times to view our closest star.
  2. The Moon. The Moon varies in brightness depending on what phase it’s in. At its full phase when it’s the brightest, it tops out at magnitude -12.6, and in a crescent phase it’s about a -6.
  3. Venus. As with all the other solar system objects, Venus varies in brightness depending on a number of factors, including how close it is to Earth and what phase it’s in. Venus can shine as brightly as -4.7 and is frequently referred to as the Evening Star or the Morning Star.
  4. Mars. Mars is the second closest planet to Earth after Venus, but it doesn’t often reach its maximum magnitude of -2.9. Its reddish-orange hue helps to distinguish it from other planets.
  5. Jupiter. This gas giant is often brighter than Mars, and at its maximum it shines almost as bright, at magnitude -2.8.
  6. Mercury. Surprise! The rarely seen Mercury shines more brightly than Saturn at its best. Mercury can reach -1.9 magnitude, but because of its position near the Sun, it’s never seen high in the night sky. Therefore its brightness is often offset by the fact that you’re viewing the planet around sunrise and sunset.
  7. Saturn. Saturn, which is stunning in a telescope sporting wide rings, is also a fairly easy catch without optical aid. At magnitude 0.7 it outshines most stars.
  8. Ganymede. The gap between the 7th and 8th brightest solar system objects is wide, with two objects butting in before the next planet. Ganymede is the largest moon of Jupiter. Keen eyes can spot it circling Jupiter when the satellite is at its brightest, approximately 4.6 magnitude.
  9. Vesta. This asteroid, which was the fourth asteroid to be discovered, can reach magnitude 5.4 at its closest approach to Earth. Because it doesn’t have a bright nearby locator, as Ganymede has with Jupiter, it’s best to watch for Vesta a couple nights in a row to see which dim “star” in the area appears to slowly move.
  10. Uranus. The seventh planet from the Sun appears at magnitude 5.5 at its best. Uranus is best picked up with the naked eye after first pinning down its location with binoculars or a telescope. It has a disk instead of a pinpoint image through an optical device and may even appear faintly bluish green.

A couple other notable solar system objects that don’t quite make our top ten list include Ceres, a bright asteroid but at magnitude 6.7, it would be a real trick to pick out with the eyes alone. Neptune at magnitude 7.7 and Pluto at 13 require optical aid. (Pluto, in particular, requires a large telescope.) Transient solar system objects, such as meteors or comets, also occasionally appear bright enough to be seen without optical aid.

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